In recent years, due to concern for the global environment, there has been a demand for structural metal materials to be light weight and to have a high strength.
With an increasing demand for safer automobile bodies, there is a further increasing demand for decreases in weight and increases in strength of automotive parts. Initial (starting) metal materials from which automotive parts are manufactured by working are required to have a strength level which is considerably higher than in the past. Therefore, high tensile strength steel sheets having a tensile strength of at least 780 MPa or even at least 900 MPa have been much used as an initial metal material for automotive parts.
As initial metal materials have increased in strength, there has been promoted a rethinking of the structure of automotive parts. For example, there is a strong demand for the development of bending techniques for highly accurate working of parts which are manufactured by continuous bending in which the bending direction varies three-dimensionally in order to manufacture high-strength automotive parts having a complicated shape.
FIGS. 4 and 5 are explanatory views schematically showing a bending apparatus 0 according to the invention which the present applicant disclosed in Patent Document 1 in response to such a demand.
A feed device 3 sends forth a metal material 1, which is supported by a support means 2 so as to be able to move in its axial direction, from an upstream side towards a downstream side. A high frequency heating coil 5 which is disposed on the downstream side of the support means 2 rapidly heats a portion of the metal material 1 to a temperature range in which quenching is possible. A water cooling device 6 which is disposed downstream of the high frequency heating coil 5 rapidly cools the metal material 1. A movable roller die 4 which is disposed on the downstream side of the water cooling device 6 has at least one pair of rolls 4a which can support the metal material 1 while feeding it. The movable roller die 4 can move three-dimensionally, thereby imparting a bending moment to the heated portion of the metal material 1 and carrying out bending.
The bending apparatus 0 can manufacture a bent product with a high operating efficiency while maintaining a sufficient bending accuracy. The resulting bent product can have a bent portion which is bent three-dimensionally and a quenched portion intermittently or continuously in the lengthwise direction and/or the circumferential direction in a plane crossing the lengthwise direction. The bending apparatus 0 can manufacture the bent product with high operating efficiency while maintaining sufficient bending accuracy.